Entity Relationships in a Document Database at CouchConf Boston

Entity Relationships in
a Document Database
MapReduce Views for SQL Users

Entity:
An object defined by its identity
and a thread of continuity[1]

1. "Entity" Domain-driven Design Community <http://domaindrivendesign.org/node/109>.

SQL Query Joining
Publishers and Books
SELECT
`publisher`.ìd`,
`publisher`.`name`,
`book`.`title`
FROM `publisher`
FULL OUTER JOIN `book`
ON `publisher`.ìd` = `book`.`publisher_id`
ORDER BY
`publisher`.ìd`,
`book`.`title`;

Joined Result Set
publisher.id publisher.name book.title
Building iPhone Apps with
oreilly O'Reilly Media
HTML, CSS, and JavaScript
CouchDB: The Deﬁnitive
Guide
DocBook: The Deﬁnitive
Guide

oreilly O'Reilly Media RESTful Web Services

Joined Result Set
Publisher (“left”)
Guide
Guide


Joined Result Set
Publisher (“left”) Book “right”
Guide
Guide


Collated Result Set
key id value

["oreilly",0] "oreilly" "O'Reilly Media"
"Building iPhone Apps with
["oreilly",1] "oreilly"
HTML, CSS, and JavaScript"
"CouchDB: The Deﬁnitive
Guide"
"DocBook: The Deﬁnitive
Guide"
["oreilly",1] "oreilly" "RESTful Web Services"

Collated Result Set
key id value

["oreilly",0] "oreilly" "O'Reilly Media" Publisher
Guide"
Guide"

Collated Result Set
key id value

["oreilly",0] "oreilly" "O'Reilly Media" Publisher
Guide"
Books
Guide"

View Result Sets
Made up of columns and rows

Every row has the same three columns:
• key
• id
• value
Columns can contain a mixture of logical data types

Embedded Entities:
Nest related entities within a document

Embedded Entities
A single document represents the “one” entity

Nested entities (JSON Array) represents the “many” entities

Simplest way to create a one to many relationship

Example: Publisher
with Nested Books
{
"_id":"oreilly",
"collection":"publisher",
"name":"O'Reilly Media",
"books":[
{ "title":"CouchDB: The Definitive Guide" },
{ "title":"RESTful Web Services" },
{ "title":"DocBook: The Definitive Guide" },
{ "title":"Building iPhone Apps with HTML, CSS,
and JavaScript" }
]
}

Map Function
function(doc) {
if ("publisher" == doc.collection) {
emit([doc._id, 0], doc.name);
for (var i in doc.books) {
emit([doc._id, 1], doc.books[i].title);
}
}
}

Result Set
key id value

Guide"
Guide"

Limitations
Only works if there aren’t a large number of related entities:
• Too many nested entities can result in very large documents
• Slow to transfer between client and server
• Unwieldy to modify
• Time-consuming to index

Related Documents:
Reference an entity by its identifier

Related Documents
A document representing the “one” entity

Separate documents for each “many” entity

Each “many” entity references its related
“one” entity by the “one” entity’s document identifier

Makes for smaller documents

Reduces the probability of document update conflicts

Example: Publisher
{
"_id":"oreilly",
"collection":"publisher",
"name":"O'Reilly Media"
}

Example: Related Book
{
"_id":"9780596155896",
"collection":"book",
"title":"CouchDB: The Definitive Guide",
"publisher":"oreilly"
}

Map Function
function(doc) {
if ("publisher" == doc.collection) {
}
if ("book" == doc.collection) {
emit([doc.publisher, 1], doc.title);
}
}

Result Set
key id value

["oreilly",1] "9780596155896"
Guide"
["oreilly",1] "9780596529260" "RESTful Web Services"
["oreilly",1] "9780596805791"
["oreilly",1] "9781565925809"
Guide"

Limitations
When retrieving the entity on the “right” side of the relationship,
one cannot include any data from the entity on the “left” side of
the relationship without the use of an additional query

Only works for one to many relationships

List of Keys:
Reference entities by their identifiers

List of Keys
A document representing each “many” entity on the “left” side
of the relationship

Separate documents for each “many” entity on the “right” side
of the relationship

Each “many” entity on the “right” side of the relationship
maintains a list of document identifiers for its related “many”
entities on the “left” side of the relationship

Example: Book
{
"_id":"9780596805029",
"title":"DocBook 5: The Definitive Guide"
}

Example: Book
{
"_id":"9781565920514",
"title":"Making TeX Work"
}

Example: Book
{
"_id":"9781565925809",
"title":"DocBook: The Definitive Guide"
}

Example: Author
{
"_id":"muellner",
"collection":"author",
"name":"Leonard Muellner",
"books":[
"9781565925809"
]
}

Example: Author
{
"_id":"walsh",
"name":"Norman Walsh",
"books":[
"9780596805029",
"9781565925809",
"9781565920514"
]
}

Map Function
function(doc) {
emit([doc._id, 0], doc.title);
}
if ("author" == doc.collection) {
emit([doc.books[i], 1], doc.name);
}
}
}

Result Set
key id value
["9780596805029",0] "9780596805029" "DocBook 5: The Deﬁnitive Guide"

["9780596805029",1] "walsh" "Norman Walsh"

["9781565920514",0] "9781565920514" "Making TeX Work"


["9781565925809",0] "9781565925809" "DocBook: The Deﬁnitive Guide"

["9781565925809",1] "muellner" "Leonard Muellner"


Map Function
function(doc) {
emit([doc._id, 1], {"_id":doc.books[i]});
}
}
}

Result Set
key id value
["muellner",0] "muellner" "Leonard Muellner"

["muellner",1] "muellner" {"_id":"9781565925809"}

["walsh",0] "walsh" "Norman Walsh"

["walsh",1] "walsh" {"_id":"9780596805029"}

["walsh",1] "walsh" {"_id":"9781565920514"}

["walsh",1] "walsh" {"_id":"9781565925809"}

Including Docs
include_docs=true
key id value doc (truncated)
["muellner",0] "muellner" … {"name":"Leonard Muellner"}
["muellner",1] "muellner" … {"title":"DocBook: The Definitive Guide"}
["walsh",0] "walsh" … {"name":"Norman Walsh"}
["walsh",1] "walsh" … {"title":"DocBook 5: The Definitive Guide"}
["walsh",1] "walsh" … {"title":"Making TeX Work"}
["walsh",1] "walsh" … {"title":"DocBook: The Definitive Guide"}

Or, we can reverse the references…

Example: Author
{
"_id":"muellner",
"name":"Leonard Muellner"
}

Example: Author
{
"_id":"walsh",
"name":"Norman Walsh"
}

Example: Book
{
"_id":"9780596805029",
"title":"DocBook 5: The Definitive Guide",
"authors":[
"walsh"
]
}

Example: Book
{
"_id":"9781565920514",
"title":"Making TeX Work",
"authors":[
"walsh"
]
}

Example: Book
{
"_id":"9781565925809",
"title":"DocBook: The Definitive Guide",
"authors":[
"muellner",
"walsh"
]
}

Map Function
function(doc) {
}
for (var i in doc.authors) {
emit([doc.authors[i], 1], doc.title);
}
}
}

Result Set
key id value
["muellner",1] "9781565925809" "DocBook: The Definitive Guide"
["walsh",1] "9780596805029" "DocBook 5: The Definitive Guide"
["walsh",1] "9781565920514" "Making TeX Work"
["walsh",1] "9781565925809" "DocBook: The Definitive Guide"

Limitations
Queries from the “right” side of the relationship cannot include
any data from entities on the “left” side of the relationship
(without the use of include_docs)

A document representing an entity with lots of relationships
could become quite large

Relationship Documents:
Create a document to represent each
individual relationship

Relationship Documents
A document representing each “many” entity on the “left” side
of the relationship

Separate documents for each “many” entity on the “right” side
of the relationship

Neither the “left” nor “right” side of the relationship contain any
direct references to each other

For each distinct relationship, a separate document includes the
document identifiers for both the “left” and “right” sides of the
relationship

Example:
Relationship Document
{
"_id":"44005f2c",
"collection":"book-author",
"book":"9780596805029",
"author":"walsh"
}

Example:
{
"_id":"44005f72",
"book":"9781565920514",
"author":"walsh"
}

Example:
{
"_id":"44006720",
"book":"9781565925809",
"author":"muellner"
}

Example:
{
"_id":"44006b0d",
"book":"9781565925809",
"author":"walsh"
}

Map Function
function(doc) {
emit([doc._id, 0], doc.title);
}
if ("book-author" == doc.collection) {
emit([doc.book, 1], {"_id":doc.author});
}
}

Result Set
key id value
["9780596805029",0] "9780596805029" "DocBook 5: The Deﬁnitive Guide"
["9780596805029",1] "44005f2c" {"_id":"walsh"}
["9781565920514",0] "9781565920514" "Making TeX Work"
["9781565920514",1] "44005f72" {"_id":"walsh"}
["9781565925809",0] "9781565925809" "DocBook: The Deﬁnitive Guide"
["9781565925809",1] "44006720" {"_id":"muellner"}
["9781565925809",1] "44006b0d" {"_id":"walsh"}

Including Docs
include_docs=true
["9780596805029",0] … … {"title":"DocBook 5: The Deﬁnitive Guide"}
["9780596805029",1] … … {"name":"Norman Walsh"}
["9781565920514",0] … … {"title":"Making TeX Work"}
["9781565920514",1] … … {"author","name":"Norman Walsh"}
["9781565925809",0] … … {"title":"DocBook: The Deﬁnitive Guide"}
["9781565925809",1] … … {"name":"Leonard Muellner"}
["9781565925809",1] … … {"name":"Norman Walsh"}

Map Function
function(doc) {
}
if ("book-author" == doc.collection) {
emit([doc.author, 1], {"_id":doc.book});
}
}

Result Set
key id value

["muellner",1] "44006720" {"_id":"9781565925809"}


["walsh",1] "44005f2c" {"_id":"9780596805029"}

["walsh",1] "44005f72" {"_id":"9781565920514"}

["walsh",1] "44006b0d" {"_id":"9781565925809"}

Including Docs
include_docs=true
["muellner",0] … … {"name":"Leonard Muellner"}
["muellner",1] … … {"title":"DocBook: The Definitive Guide"}
["walsh",0] … … {"name":"Norman Walsh"}
["walsh",1] … … {"title":"DocBook 5: The Definitive Guide"}
["walsh",1] … … {"title":"Making TeX Work"}
["walsh",1] … … {"title":"DocBook: The Definitive Guide"}

Limitations
Queries can only contain data from the “left” or “right” side of the
relationship (without the use of include_docs)

Maintaining relationship documents may require more work

Document Databases Compared
to Relational Databases
Document databases have no tables (and therefore no columns)

Indexes (views) are queried directly, instead of being used to
optimize more generalized queries

Result set columns can contain a mix of logical data types

No built-in concept of relationships between documents

Related entities can be embedded in a document, referenced from
a document, or both

Caveats
No referential integrity

No atomic transactions across document boundaries

Some patterns may involve denormalized (i.e. redundant) data

Data inconsistencies are inevitable (i.e. eventual consistency)

Consider the implications of replication—what may seem
consistent with one database may not be consistent across nodes
(e.g. referencing entities that don’t yet exist on the node)

Additional Techniques
Use the startkey and endkey parameters to retrieve one entity and
its related entities:
startkey=["9781565925809"]&endkey=["9781565925809",{}]

Define a reduce function and use grouping levels

Use UUIDs rather than natural keys for better performance

Use the bulk document API when writing Relationship Documents

When using the List of Keys or Relationship Documents patterns,
denormalize data so that you can have data from the “right” and
“left” side of the relationship within your query results

Cheat Sheet
Embedded Related Relationship
List of Keys
Entities Documents Documents

One to Many ✓ ✓
Many to Many ✓ ✓
<= N* Relations ✓ ✓
> N* Relations ✓ ✓

* where N is a large number for your system

http://oreilly.com/catalog/9781449303129/ http://oreilly.com/catalog/9781449303433/

Entity Relationships in a Document Database at CouchConf Boston

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